The present invention relates to x-ray machine calibration devices and, specifically, to calibration devices for a computed tomography system.
In one method of computed tomography, a patient is supported for being translated along a longitudinal axis which coincides with the center of rotation of a rotatable gantry which has an x-ray source on one side of the center of rotation and an x-ray detector on the other side of the center of rotation. A fan-shaped x-ray beam that is thin in the longitudinal direction is projected through the patient as the gantry rotates so that the detector may develop signals indicative of x-ray transmission characteristics along a plurality of paths through the patient undergoing examination. Analog signals representative of x-ray attenuation by all of the volume elements in a layer of the patient at various rotational angles are then converted to digital signals which are used by a computer to produce signals that may thereafter be used to produce a reconstructed image of the layer. The reconstructed image of the radiation attenuation coefficients may be displayed in gray tones on the screen of a visual display.
One of the primary problems associated with computed tomography systems is proper calibration. The computed tomography system is quite complex and is subject to occasional interference and signal drift which may result in a slight shift in performance.
Another problem associated with system calibration is due to a phenomenon referred to as "beam hardening". The radiation attenuation coefficient is a property of every material and expresses the radiation absorption properties of the material at a specific x-ray energy. This attenuation coefficient is usually related to the attenuation coefficient of water and is referred to as a CT number expressed in "H-units". The CT numbers are relative and have reference points of -1000 for air and 0 for water with all other materials expressed as + or - a specific number of H units. Computed tomography radiation sources project polychromatic x-rays having energies which vary from 30 KeV to 120 KeV. The range of x-ray energies can produce a shift in CT numbers due to "hardening" of the ray spectrum as the x-rays penetrate the body. A shift in CT numbers can also be produced by repositioning the patient in a different location relative to the beam so that the patient is scanned by a different portion of the beam.
The attenuation coefficients and corresponding gray tones produced by the computed tomography system are relative and must be calibrated to known values. The system is usually calibrated on a regular basis by scanning a set of known reference samples mounted in what is referred to as a calibration phantom. Prior art phantoms are available in a number of variations, some being plastic replicas of the human body, or specific portions thereof, while others consist of actual human bones cast in plastics. Recent phantoms include a set of reference samples having known attenuation coefficients surrounded by a water medium housed within a plastic vessel. Phantoms used to calibrate computed tomography systems are usually cylindrical discs having a diameter ranging from 20 to 42 cm and having a thickness of approximately 10 cm. The phantoms are scanned in the same manner as the patient, and the resulting attenuation coefficients of the images of the system can be compared to the reference values of the phantom. Phantoms can also be used to correlate images from calibrated machines to analyze and interpret the images. A particular problem in utilizing conventional phantoms is that they cannot be used simultaneously with the analysis of a patient and that the calibration cannot be assured between applications of the phantom.
Accordingly, the object of the present invention is to provide a CT system with a set of reference samples having known radiation attenuation coefficients which appear in each reconstructed image from which to calibrate the system and to correlate the images of the patient.